I recently renewed the batteries in the infrared remote control key fob of my 1995 500SL. I followed the instructions in the owner's manual and searched for help here on in the forums, without any joy.
My system came back to life only after aiming the transmitter at a receiver, unlocking straight away and then starting the car. I haven't seen the idea of having to start the car mentioned elsewhere, but it certainly appeared to be the essential step in my case.
Here's hoping this might help someone else.